1. Field of the Invention
This invention pertains to micronized grinding apparatus and is directed to means for improving the ability of such apparatus to produce a product having a substantially uniform micronized product output.
2. Description of the Prior Art
A prevailing construction of roller grinding mill comprises a frame defining a grinding chamber provided with a bull ring against which a set of orbiting rollers crushes the material by application of controlled centrifugal force exerted by the rolls as they orbit and also roll relative to the bull ring. An air bustle surrounds the grinding chamber portion of the frame and generally radially oriented ports open between the bustle and grinding chamber below the bull ring for fluidizing and effecting movement of material upwardly into the orbit of the grinding rollers. Such a construction is seen in Williams U.S. Pat. No. 4,022,387 of May 10, 1977, as well as in Williams U.S. Pat. No. 3,337,142 of Aug. 22, 1967.
Another type of roller pulverizing mill is seen in Hardinge U.S. Pat. No. 2,909,330 of Oct. 20, 1959 in which the grinding rollers do not orbit but are caused to roll over material fed upon a rotating plate or fed into a bowl. Air is delivered below the rotating plate or bowl and passes up about the outside thereof to lift material which reaches the periphery of the plate or bowl. Other similar mills are seen in Crites U.S. Pat. Nos. 2,601,954 of July 1, 1952 and 2,815,903 of Dec. 10, 1957.
A problem evident in the prior art is that material fed into the grinding chamber is allowed to set up an eccentric distribution so vibration occurs which is detrimental to such apparatus. Uneven grinding and crushing results from this vibration, a high noise level is reached, and capacity of the mill output is reduced. Fine ground material such as coal, has a mass which allows ready transportation in the air stream behind the grinding rolls and out of the top of the grinding chamber.